The above drawing may not be correct for your car.
I was helping a kid on FABO who posted some pictures of his 1976 A-body. These were very helpful. Comparing his pics to the wiring diagrams from a factory service manual, it looks like at least some 76 models got an externally shunted ammeter.

If you don't see a drop in voltage between Alt Output stud and Battery during charging, the main wires ( A1 & R6) must be well connected.*

That leaves the connections from the main power line through to the voltage regulator as the places that must be causing a 1.5 V drop.

*Voltage drops only occur when current is flowing through a resistance, so checking when the battery is fully charged, or when the engine is off will not reveal anything.